At all other places I know, each one given point, as defined by latitude/longitude/elevation, belongs to one and exactly one part of airspace, with one and exactly one service in charge. So, depending on where you are, you call the relevant service and they tell you what QNH to set. Solved. Why the UK absolutely wants all this vagueness and confusion is beyond me.
BS
When flying outside controlled airspace at or below 4 500 FT AMSL, the altimeter shall be set to the regional QNH given on pilot's request by Brussels FIC, Semmerzake ATCC or by any ATC unit. The regional QNH is the lowest of the QNH values for the following stations: EBAW, EBBE, EBBR, EBCI, EBCV, EBFS, EBBL, EBFN, EBLG, ELLX, EBOS, EBSH and EBSP.
How does that differ from this?
Altimeter Setting Regions (ASR). To make up for any lack of stations reporting actual QNH, the UK has been divided into a
number of ASRs for each of which the National Meteorological Office calculates the lowest forecast QNH (Regional Pressure
Setting) for each hour. These values are available hourly for the period H+1 to H+2 and may be obtained from all aerodromes having an Air Traffic Service, from London AC (Swanwick) and Scottish AC (Prestwick), or by telephone.